Heat exchangers, such as the steam generators of pressurized-water nuclear reactors, generally comprise a tube plate of great thickness, through which pass tubes which are flush with one of the faces of this tube plate forming its entry face and which project on the other side of the tube plate, the exit face of which is in contact with the interior of the steam generator receiving the feed water to be evaporated. The entry face of the tube plate opens into the inner volume of a two-part water box ensuring the distribution and recovery of the pressurized water circulating in the tubes and carrying the heat for the heating and evaporation of the feed water. Each of the steam-generator tubes, which is bent in its upper part, has two ends opening into the respective parts of the water box.
The tubes of the steam generator form a bundle and are fitted inside holes of suitable diameter which pass through the tube plate virtually over the entire thickness of this plate. The tube bundle arranged in that part of the steam generator delimited by the exit face of the tube plate is retained inside the casing of the steam generator by means of spacer plates arranged at uniform distances from one another over the height of the bundle and having a network of holes corresponding to the transverse positions of the tubes of the bundle.
During operation of the reactor, some zones of the tubes of the bundle are subjected to increased local corrosion both from the inner surface of the tube in contact with the pressurized water and from the outer surface in contact with the feed water. This increased corrosion is usually attributable to a concentration of high residual stresses or to the accumulation of corrosive products in contact with the outer surface of the tube.
In particular, the transition zone between the region of the tube deformed as a result of fitting inside the tube plate and the non-deformed region of the tube is subjected to accelerated corrosion attributable to the concentration of stresses and to the accumulation of corrosive products on the upper exit face of the tube plate.
When the maintenance of the steam generators of a nuclear reactor is carried out, the reactor being shut down, it is necessary to conduct checks of the tubes, especially in order to ascertain their corrosion resistance in the transition zone after a certain period of service. These checks may require the extraction of a tube portion surrounding the transition zone. This extraction is useful only if it can be carried out without any deformation or change of surface state of the tube in the zone to be inspected.
French Patent No. 2,591,744, assigned to the present applicant discloses a process and apparatus for the extraction of a heat-exchanger tube portion, making it possible to prevent any deformation of change or surface state of the tube portion to be extracted. The tube portion is previously separated from the rest of the tube by cutting the wall of the tube in a zone located at the upper end of the portion to be extracted. A sleeve is then inserted into the tube from the entry face of the tube plate and over such a length that the sleeve covers the tube over the entire passage through the tube plate and over the entire zone which will be inspected after the extraction of the tube portion. A junction between the tube and the sleeve is made in two zones located on either side of the zone to be inspected, by means of glue which creates a connection of high mechanical resistance between the inner surface of the tube and the outer surface of the sleeve. Finally, axial pull is exerted on the sleeve from that face of the tube plate with which the tube is flush, in order to extract the tube portion from the tube plate.
During extraction, the zone of the tube to be inspected is not subjected to any stress, the sleeve ensuring that this zone is bridged in its entirety.
The sleeve is fitted into the tube by means of a device comprising a glue-injecting syringe which is engaged and fastened in the sleeve in a desired position, before the sleeve is introduced into the tube. The sleeve has orifices to allow the glue to pass through in the two bonding zones, the spacing of which corresponds to the length of the zone of the tube to be inspected.
The sleeve and the syringe are introduced into the tube via the entry face of the tube plate and are then installed and retained in the desired position inside the tube during injection of the glue. The syringe is subsequently extracted from the sleeve before the glue has set completely.
When the glue has hardened sufficiently, a pull is exerted on the sleeve by means of a suitable device, so as to extract the tube portion from the tube plate.
The operations of fitting the sleeve and injecting the glue can be carried out by an operator from the water box of the steam generator.
The process and device described in French Patent No. 2,591,744 are entirely suitable for the extraction of a tube portion consisting of the end part of the tube fitted in the tube plate and the immediately adjacent part of the tube corresponding to the transition zone. In fact, in this case, the length of the sleeve is only very slightly greater than the thickness of the tube plate, and therefore the deflection within the water box is sufficient to allow the rigid assembly consisting of the sleeve and of the syringe to be introduced into the tube.
However, it may prove necessary to conduct the check of the tubes by extracting a portion not only in the transition zone, but also in zones more remote from the ends of the tube, for example in the region of the first spacer plate of the steam generator. In this case, the process and device described cannot be used because the extraction sleeve and the syringe forming a rigid assembly would have a considerable length incompatible with the possible deflection within the water box. This is especially true of large bends, i.e., of the tubes located furthest to the outside of the bundle, the hemispherical water box having a small height in line with these tubes.
To date, no process and apparatus is known for extracting a steam-generator tube portion in a zone remote from the ends of the tube, for example in the vicinity of a spacer plate for the retention of the bundle, in such a way as to preserve the state of the portion to be extracted and prevent any breakage of the tube during extraction as a result of cracking.